Wood fiber reinforced polypropylene composites of different fiber contents without any treatment were prepared, and tensile tests were carried out on injection molded specimens. With increasing fiber content a decrease of the tensile strength was experienced. The weak adhesion at the fiber-matrix interface and the typical composite failures can be seen on SEM pictures. During the tests acoustic emission was monitored to get more information about the damage mechanism. From the AE counts distribution it can be concluded that the maximum number of AE counts decreases simultaneously with the tensile strength in case of the different composites.